The present invention relates to photographic printing systems. In particular, the present invention is an improved method and apparatus for controlling exposure times.
Photographic printers produce color or black and white prints or transparencies from photographic film originals (generally negatives). High intensity light is passed through the film and imaged on the photosensitive print medium (film or paper). The photographic emulsion layer on the print paper or film is exposed and subsequently processed to produce a print or transparency of the scene contained in the original.
In order to increase efficiency and minimize time required to fill customer orders, high speed printers have been developed in which many exposures are made on a single roll of print paper. After the exposures are made, the roll is removed from the printer, is photoprocessed to produce prints, and is cut into individual prints. The prints are then sorted by customer order and ultimately packaged and sent to the customer.
A critical portion of a photographic printer is the exposure time control, which controls the duration of the exposure of the photosensitive medium in order to assure that the image on the photosensitive medium is properly exposed. The exposure time control may utilize inputs from several different sources in order to determine the proper duration of the exposure. For example, most automatic printers use large area transmission density (LATD) sensors to sample the light transmitted by the negative either prior to or during the exposure. Control of the exposure time is determined using a method known as "integration to grey". In addition, many automatic printers include an automatic density correction (ADC) or color scanning station which scans the negative prior to printing and corrects the exposure time in the event of an abnormality in illumination of the negative known as "subject failure". Finally, the operator may enter density or color correction signals from the operator control panel. Based upon some or all of these input signals, the exposure time control determines the proper exposure time for each of the color channels or for one black and white channel.
The significant advances in digital electronics and digital computers in recent years has led to the development of computer control of photographic printers. One of the functions controlled by the computer (which has typically been a minicomputer) is the exposure time control function. In the past, a significant amount of computing time has been dedicated to the control of the exposure time.